In a micro electro mechanical system (MEMS), the development of a sacrificial layer technique has become a key factor for manufacturing a suspended structure, such as a cantilever, a beam, a membrane, a channel, a cavity, a joint or hinge, a link, a crank, a gear or a rack, to name a few. A structure release etching process is adapted for removing a sacrificial layer, so a structure of a structure release in a micro electro mechanical system has a critical influence on the process of removing the sacrificial layer.
A conventional structure release etching process is first introduced with an interference display cell as an example. The interference display cell, a kind of a micro electro mechanical system, is used to fabricate a planar display. Planar displays have great superiority in the portable display device and limited-space display market because they are lightweight and small. To date, in addition to liquid crystal displays (LCD), organic electro-luminescent displays (OLED), and plasma display panels (PDP), a mode of optical interference display is another option for planar displays.
FIG. 1A and FIG. 1B illustrate a method for manufacturing a conventional optical interference display cell. Referring to FIG. 1A, a first electrode 110 and a sacrificial layer 111 are formed in sequence on a transparent substrate 109, and openings 112, each suitable for forming a supporter therein, are formed in the first electrode 110 and the sacrificial layer 111. Then, a supporter 106 is formed in each of the openings 112. Next, an electrode 114 is formed on the sacrificial layer 111 and the supporter 106. Subsequently, referring to FIG. 1B, the sacrificial layer 111 shown in FIG. 1A is removed by a release etching process to form a cavity 116, which is located in the position of the sacrificial layer 111, and the length D of the cavity 116 is the thickness of the sacrificial layer 111.
In a micro electro mechanical process, a micro suspended structure is fabricated by the use of a sacrificial layer. A suspended movable microstructure is fabricated by a selective etching between a device structure layer and the sacrificial layer to remove the sacrificial layer and leave the structure layer, and this process is called a structure release etching. The difference between the structure release etching process and an IC process is that in the structure release etching process, the selective etching is an isotropic etching, so that an undercut or an under etching is formed in the structure layer for smooth separation of the structure layer and the substrate.
The most popular structure release etching process is a wet structure release process. In the wet structure release process, a rinsing step and a drying step usually have to be performed after etching, and a microstructure can substantially be suspended above the substrate. However, during the wet structure release process, it is quite easy for the structure and the substrate to stick together, thereby resulting in failure of the device. A dry etching process using xenon difluoride (XeF2) as an etchant can be used to solve the problems due to the wet etching process.
Xenon difluoride is in a solid state at normal temperature and normal pressure, and is sublimated into the gaseous state at low pressure. Xenon difluoride has a high etching rate for silicon materials such as monocrystalline silicon, polysilicon and amorphous silicon, and some metals such as molybdenum (Mo), molybdenum alloy and so on. Xenon is an inert gas, and xenon difluoride is quite unstable. The etching mechanism of xenon difluoride is that two fluorine free radicals are brought to the reaction positions by xenon, and when xenon difluoride contacts the material to be etched, xenon difluoride decomposes to release these two fluorine free radicals. Because the isotropic etching effect of xenon difluoride is great, xenon difluoride has an excellent capacity for lateral etching. In a micro electro mechanical system process, xenon difluoride is used as an etchant to remove a sacrificial layer in a structure release etching process. Typically, since the activity of xenon difluoride is quite high, i.e., the activation energy of the decomposition of xenon difluoride into fluorine free radicals is quite low, and a reaction occurs almost immediately as soon as xenon difluoride contacts the material to be etched even at room temperature. Therefore, raising the etching temperature can hardly increase the etching rate of xenon difluoride. A xenon difluoride etching process is typically conducted at a temperature lower than 70° C.
FIG. 2 illustrates a top view of a conventional optical interference display cell 200. The optical interference display cell 200 includes separation structures 202, as indicated by dotted lines 2021, located on two opposite sides of the optical interference display cell 200, and supporters 204 located on the other two opposite sides of the optical interference display cell 200. The separation structures 202 and the supporters 204 are located between two electrodes. There are gaps between the supporters 204, as well as between the supporters 204 and the separation structures 202. The gaseous xenon difluoride permeates through the gaps and etches a sacrificial layer (not shown in FIG. 2). The etching rate of a structure release etching process using an etchant of the gaseous xenon difluoride depends on the material of the sacrificial layer desired to be etched. Typically, the etching rate exceeds 10 micrometers per minute, and can be up to 20–30 micrometers per minute for some materials. So far as the size of the present optical interference display cell is concerned, one structure release etching process only takes dozens of seconds to 3 minutes.
Although the structure release etching process performed with the etchant of gaseous xenon difluoride has the aforementioned advantages, the structure release etching process has a drawback: the cost can't be reduced due to the character of xenon difluoride itself. Xenon difluoride is expensive, is particularly sensitive to moisture and is unstable. When xenon difluoride contacts moisture, hydrogen fluoride is produced. Hydrogen fluoride is not only dangerous, but also reduces efficiency of etching. Besides, the structure release etching process performed using xenon difluoride as an etchant is rarely found in semiconductor processes and typical planar display processes, so etchers that are well developed in current semiconductor processes and liquid crystal display processes are unsuitable for the structure release etching process using xenon difluoride etchant. Most of the main manufacturing processes of the optical interference display can continue using conventional semiconductor or planar display processing equipment, but the structure release etching process needs a totally different apparatus design. To reorganize and consolidate the processing equipments would be an obstacle to the development and mass production of the optical interference display.